Washington — The US government's unprecedented efforts to rescue mortgage giants Fannie Mae and Freddie Mac are a reminder that the most important things presidents do are often not action items from preset agendas but reactive moves to events unforeseen.

Eight years ago, George W. Bush didn't campaign on a proposal to save the housing market – or what he'd do if terrorists attacked the US. In 1960, John F. Kennedy didn't run on his ability to respond to Soviet missiles in Cuba, either.

In this election year, the implication is obvious: It's not just about the four-point plans, or the "vision thing," as George H.W. Bush referred to over­arching agendas. Voters might best judge a candidate's flexibility, relevant experience, and habits of mind.

"Lots of times, you look back at the four or eight years a president served ... and see that [their] success or failure was determined by things they didn't necessarily talk about as a candidate," says Stephen Hess, a governance expert at the Brookings Institution in Washington and author of the forthcoming book "What Do We Do Now? A Workbook for the President-Elect."

This phenomenon may be most prevalent in foreign policy. The world has a way of grabbing the attention of even the most domestic-oriented of Oval Office occupants.

Candidate George W. Bush famously promised a "humble" foreign policy and expressed aversion to nation-building. But the judgment of historians on his time in office may well rest on the final outcome of his invasion of Iraq, arguably America's most intense involvement in nation-building since the end of World War II.

"There is a long history of presidents talking about being humble in foreign policy and then getting the US intensely involved in world affairs," says Mr. Lichtman.

But unforeseen domestic circumstances have also caused many presidents to take actions that at the time seemed at odds with their professed ideology.

Republican Richard Nixon professed disdain for wage and price controls on the economy, for instance. Yet – stuck in an unpopular and expensive war, and fearful of inflation – he imposed them in 1971.

For the market-oriented Bush administration, its promise of support for Fannie Mae and Freddie Mac might rank as an about-face. Given the current turbulence in the mortgage market, the administration might not have much choice but to prop up institutions that together own or guarantee half of the home loans in the US.

So what happened? In a campaign, notes presidential historian George C. Edwards III of Texas A&M University in College Station, candidates try to present a clear narrative of themselves. Ideology can be a crucial part of this story arc.

That doesn't mean ideology is unimportant. It is still the best indicator of an administration's general thrust, says Edwards. The Bush administration generally has been free market, whatever the implications of its Fannie Mae and Freddie Mac intervention.

It's slavish devotion to ideology that perhaps can cause the most trouble.

"The president in general has to have the ability to be nuanced," says Edwards.

So what's a voter to do? They can't judge how candidates will react to unforeseen crises any more than they can pick next week's direction of the stock market.

One thing they can do is look at a candidate's experience, says Mr. Hess of Brookings. That's relevant experience – not years in office, per se.

This is easier to do if a candidate has had executive experience, according to Hess. Governors, university presidents, business leaders – all have had to react to sudden developments at one time or another.

Legislators generally haven't. And this year, the two candidates, of course, are senators. Washington legislators sometimes have difficulty adjusting to the pace of executive authority, says Hess – making clear that in this case he is not necessarily referring to Sen. John McCain, whose legislative career as a self-professed maverick has been somewhat unusual.